Probe-based, such as catheter-based, optical systems are applicable to a number of diagnostic and therapeutic medical applications. Optical coherence tomography is used to provide spatial resolution, enabling the imaging of internal structures. Spectroscopy is used to characterize the composition of structures, enabling the diagnosis of medical conditions, by differentiating between cancerous, dysplastic, and normal tissue structures, for example. Ablation systems are used to remove or destroy structures within the body to address various diseases, such as tachycardias, tumors, and coronary artery disease, in another example of a probe-based optical system.
For example, in one specific spectroscopic application, an optical source, such as a tunable laser, is used to access or scan a spectral band of interest, such as a scan band in the near infrared or 750 nanometers (nm) to 2.5 micrometers (μm). The generated light is used to illuminate tissue in a target area in vivo using the catheter. Diffusely reflected light resulting from the illumination is then collected and transmitted to a detector system, where a spectral response is resolved. The response is used to assess the state of the tissue.
This system can be used to diagnosis atherosclerosis, and specifically identify atherosclerotic lesions or plaques. This is an arterial disorder involving the intimae of medium- or large-sized arteries, including the aortic, carotid, coronary, and cerebral arteries.
Diagnostic systems based on Raman and fluorescence based schemes have also been proposed. Other wavelengths, such as visible, have also been suggested.
The environment in which the spectra are collected, however, creates problems. Due to the presence of intervening fluid, such as blood in the case of probes inserted into blood vessels, the spectral signals related to the properties of the tissue are overwhelmed. Thus, more complicated methods such as chemometrics are used to extract the spectra of the vessel walls in the presence of noise from blood, for example.
Chemometrics is the science of relating measurements made on a chemical system or process to the state of the system via application of mathematical or statistical methods. It can be used to predict the properties, such as chemical composition, of structures based on their spectral response. Mathematical manipulations such as linear regression of the spectral band of interest and classic least squares and inverse least squares and other multivariate analysis tools are available for building quantitative calibrations as well as qualitative models for discriminant analysis.